Boolean literals.

lib/src/ast/sass/expression.dart

@@ -5,6 +5,7 @@
 import '../../visitor/expression.dart';
 import 'node.dart';
 
+export 'expression/boolean.dart';
 export 'expression/identifier.dart';
 export 'expression/interpolation.dart';
 export 'expression/list.dart';

lib/src/ast/sass/expression/boolean.dart

@@ -0,0 +1,21 @@
+// Copyright 2016 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'package:source_span/source_span.dart';
+
+import '../../../visitor/expression.dart';
+import '../expression.dart';
+
+class BooleanExpression implements Expression {
+  final bool value;
+
+  final SourceSpan span;
+
+  BooleanExpression(this.value, {this.span});
+
+  /*=T*/ visit/*<T>*/(ExpressionVisitor/*<T>*/ visitor) =>
+      visitor.visitBooleanExpression(this);
+
+  String toString() => value.toString();
+}

lib/src/parser.dart

@@ -394,11 +394,16 @@ class Parser {
   Expression _identifierLike() {
     // TODO: url(), functions
     var identifier = _interpolatedIdentifier();
-    if (identifier.asPlain == "not") {
+    switch (identifier.asPlain) {
+      case "not":
         _ignoreComments();
         return new UnaryOperatorExpression(
-          UnaryOperator.not, _singleExpression());
+            UnaryOperator.not, _singleExpression(), span: identifier.span);
-    } else {
+
+      case "true": return new BooleanExpression(true, span: identifier.span);
+      case "false": return new BooleanExpression(false, span: identifier.span);
+
+      default:
         return new IdentifierExpression(identifier);
     }
   }

lib/src/visitor/expression.dart

@@ -8,6 +8,7 @@ class ExpressionVisitor<T> {
   T visit(Expression expression) => expression.visit(this);
 
   T visitVariableExpression(VariableExpression node) => null;
+  T visitBooleanExpression(BooleanExpression node) => null;
 
   T visitUnaryOperatorExpression(UnaryOperatorExpression node) {
     node.operand.visit(this);
@@ -19,13 +20,6 @@ class ExpressionVisitor<T> {
     return null;
   }
 
-  T visitInterpolationExpression(InterpolationExpression node) {
-    for (var value in node.contents) {
-      if (value is Expression) value.visit(this);
-    }
-    return null;
-  }
-
   T visitListExpression(ListExpression node) {
     for (var expression in node.contents) {
       expression.visit(this);
@@ -37,4 +31,11 @@ class ExpressionVisitor<T> {
     visitInterpolationExpression(node.text);
     return null;
   }
+
+  T visitInterpolationExpression(InterpolationExpression node) {
+    for (var value in node.contents) {
+      if (value is Expression) value.visit(this);
+    }
+    return null;
+  }
 }

lib/src/visitor/expression/perform.dart

@@ -36,6 +36,9 @@ class PerformExpressionVisitor extends ExpressionVisitor<Value> {
   Identifier visitIdentifierExpression(IdentifierExpression node) =>
       new Identifier(visitInterpolationExpression(node.text).text);
 
+  Boolean visitBooleanExpression(BooleanExpression node) =>
+      new Boolean(node.value);
+
   SassString visitInterpolationExpression(InterpolationExpression node) {
     return new SassString(node.contents.map((value) {
       if (value is String) return value;